Stacked organic photosensitive devices

ABSTRACT

A device is provided having a first electrode, a second electrode, a first photoactive region having a characteristic absorption wavelength λ 1  and a second photoactive region having a characteristic absorption wavelength λ 2 . The photoactive regions are disposed between the first and second electrodes, and further positioned on the same side of a reflective layer, such that the first photoactive region is closer to the reflective layer than the second photoactive region. The materials comprising the photoactive regions may be selected such that λ 1  is at least about 10% different from λ 2 . The device may further comprise an exciton blocking layer disposed adjacent to and in direct contact with the organic acceptor material of each photoactive region, wherein the LUMO of each exciton blocking layer other than that closest to the cathode is not more than about 0.3 eV greater than the LUMO of the acceptor material.

FIELD OF THE INVENTION

The present invention generally relates to organic photosensitiveoptoelectronic devices. More specifically, it is directed to organicphotosensitive optoelectronic devices having increased efficiency.

BACKGROUND OF THE INVENTION

Optoelectronic devices rely on the optical and electronic properties ofmaterials to either produce or detect electromagnetic radiationelectronically or to generate electricity from ambient electromagneticradiation.

Photosensitive optoelectronic devices convert electromagnetic radiationinto electricity. Solar cells, also called photovoltaic (PV) devices,are a type of photosensitive optoelectronic device that is specificallyused to generate electrical power. PV devices, which may generateelectrical energy from light sources other than sunlight, can be used todrive power consuming loads to provide, for example, lighting, heating,or to power electronic circuitry or devices such as calculators, radios,computers or remote monitoring or communications equipment. These powergeneration applications also often involve the charging of batteries orother energy storage devices so that operation may continue when directillumination from the sun or other light sources is not available, or tobalance the power output of the PV device with a specific application'srequirements. As used herein the term “resistive load” refers to anypower consuming or storing circuit, device, equipment or system.

Another type of photosensitive optoelectronic device is a photoconductorcell. In this function, signal detection circuitry monitors theresistance of the device to detect changes due to the absorption oflight.

Another type of photosensitive optoelectronic device is a photodetector.In operation a photodetector is used in conjunction with a currentdetecting circuit which measures the current generated when thephotodetector is exposed to electromagnetic radiation and may have anapplied bias voltage. A detecting circuit as described herein is capableof providing a bias voltage to a photodetector and measuring theelectronic response of the photodetector to electromagnetic radiation.

These three classes of photosensitive optoelectronic devices may becharacterized according to whether a rectifying junction as definedbelow is present and also according to whether the device is operatedwith an external applied voltage, also known as a bias or bias voltage.A photoconductor cell does not have a rectifying junction and isnormally operated with a bias. A PV device has at least one rectifyingjunction and is operated with no bias. A photodetector has at least onerectifying junction and is usually but not always operated with a bias.As a general rule, a photovoltaic cell provides power to a circuit,device or equipment, but does not provide a signal or current to controldetection circuitry, or the output of information from the detectioncircuitry. In contrast, a photodetector or photoconductor provides asignal or current to control detection circuitry, or the output ofinformation from the detection circuitry but does not provide power tothe circuitry, device or equipment.

Traditionally, photosensitive optoelectronic devices have beenconstructed of a number of inorganic semiconductors, e.g., crystalline,polycrystalline and amorphous silicon, gallium arsenide, cadmiumtelluride and others. Herein the term “semiconductor” denotes materialswhich can conduct electricity when charge carriers are induced bythermal or electromagnetic excitation. The term “photoconductive”generally relates to the process in which electromagnetic radiant energyis absorbed and thereby converted to excitation energy of electriccharge carriers so that the carriers can conduct, i.e., transport,electric charge in a material. The terms “photoconductor” and“photoconductive material” are used herein to refer to semiconductormaterials which are chosen for their property of absorbingelectromagnetic radiation to generate electric charge carriers.

PV devices may be characterized by the efficiency with which they canconvert incident solar power to useful electric power. Devices utilizingcrystalline or amorphous silicon dominate commercial applications, andsome have achieved efficiencies of 23% or greater. However, efficientcrystalline-based devices, especially of large surface area, aredifficult and expensive to produce due to the problems inherent inproducing large crystals without significant efficiency-degradingdefects. On the other hand, high efficiency amorphous silicon devicesstill suffer from problems with stability. Present commerciallyavailable amorphous silicon cells have stabilized efficiencies between 4and 8%. More recent efforts have focused on the use of organicphotovoltaic cells to achieve acceptable photovoltaic conversionefficiencies with economical production costs.

PV devices may be optimized for maximum electrical power generationunder standard illumination conditions (i.e., Standard Test Conditionswhich are 1000 W/m², AM1.5 spectral illumination), for the maximumproduct of photocurrent times photovoltage. The power conversionefficiency of such a cell under standard illumination conditions dependson the following three parameters: (1) the current under zero bias,i.e., the short-circuit current I_(SC), (2) the photovoltage under opencircuit conditions, i.e., the open circuit voltage V_(OC), and (3) thefill factor, ff.

PV devices produce a photo-generated current when they are connectedacross a load and are irradiated by light. When irradiated underinfinite load, a PV device generates its maximum possible voltage, Vopen-circuit, or V_(OC). When irradiated with its electrical contactsshorted, a PV device generates its maximum possible current, Ishort-circuit, or I_(SC). When actually used to generate power, a PVdevice is connected to a finite resistive load and the power output isgiven by the product of the current and voltage, I×V. The maximum totalpower generated by a PV device is inherently incapable of exceeding theproduct, I_(SC)×V_(OC). When the load value is optimized for maximumpower extraction, the current and voltage have the values, I_(max) andV_(max), respectively.

A figure of merit for PV devices is the fill factor, ff, defined as:ff={I _(max) V _(max) }/{I _(SC) V _(OC)}  (1)where ff is always less than 1, as I_(SC) and V_(OC) are never obtainedsimultaneously in actual use. Nonetheless, as ff approaches 1, thedevice has less series or internal resistance and thus delivers agreater percentage of the product of I_(SC) and V_(OC) to the load underoptimal conditions. Where P_(inc) is the power incident on a device, thepower efficiency of the device, η_(P), may be calculated by:η_(P) =ff*(I _(SC) *V _(OC))/P _(inc)

When electromagnetic radiation of an appropriate energy is incident upona semiconductive organic material, for example, an organic molecularcrystal (OMC) material, or a polymer, a photon can be absorbed toproduce an excited molecular state. This is represented symbolically asS₀+hv→S₀*. Here S₀ and S₀* denote ground and excited molecular states,respectively. This energy absorption is associated with the promotion ofan electron from a bound state in the HOMO energy level, which may be aπ-bond, to the LUMO energy level, which may be a π*-bond, orequivalently, the promotion of a hole from the LUMO energy level to theHOMO energy level. In organic thin-film photoconductors, the generatedmolecular state is generally believed to be an exciton, i.e., anelectron-hole pair in a bound state which is transported as aquasi-particle. The excitons can have an appreciable life-time beforegeminate recombination, which refers to the process of the originalelectron and hole recombining with each other, as opposed torecombination with holes or electrons from other pairs. To produce aphotocurrent the electron-hole pair becomes separated, typically at adonor-acceptor interface between two dissimilar contacting organic thinfilms. If the charges do not separate, they can recombine in a geminantrecombination process, also known as quenching, either radiatively, bythe emission of light of a lower energy than the incident light, ornon-radiatively, by the production of heat. Either of these outcomes isundesirable in a photosensitive optoelectronic device.

Electric fields or inhomogeneities at a contact may cause an exciton toquench rather than dissociate at the donor-acceptor interface, resultingin no net contribution to the current. Therefore, it is desirable tokeep photogenerated excitons away from the contacts. This has the effectof limiting the diffusion of excitons to the region near the junction sothat the associated electric field has an increased opportunity toseparate charge carriers liberated by the dissociation of the excitonsnear the junction.

To produce internally generated electric fields which occupy asubstantial volume, the usual method is to juxtapose two layers ofmaterial with appropriately selected conductive properties, especiallywith respect to their distribution of molecular quantum energy states.The interface of these two materials is called a photovoltaicheterojunction. In traditional semiconductor theory, materials forforming PV heterojunctions have been denoted as generally being ofeither n or p type. Here n-type denotes that the majority carrier typeis the electron. This could be viewed as the material having manyelectrons in relatively free energy states. The p-type denotes that themajority carrier type is the hole. Such material has many holes inrelatively free energy states. The type of the background, i.e., notphoto-generated, majority carrier concentration depends primarily onunintentional doping by defects or impurities. The type andconcentration of impurities determine the value of the Fermi energy, orlevel, within the gap between the highest occupied molecular orbital(HOMO) energy level and the lowest unoccupied molecular orbital (LUMO)energy level, called the HOMO-LUMO gap. The Fermi energy characterizesthe statistical occupation of molecular quantum energy states denoted bythe value of energy for which the probability of occupation is equal to½. A Fermi energy near the LUMO energy level indicates that electronsare the predominant carrier. A Fermi energy near the HOMO energy levelindicates that holes are the predominant carrier. Accordingly, the Fermienergy is a primary characterizing property of traditionalsemiconductors and the prototypical PV heterojunction has traditionallybeen the p-n interface.

The term “rectifying” denotes, inter alia, that an interface has anasymmetric conduction characteristic, i.e., the interface supportselectronic charge transport preferably in one direction. Rectificationis associated normally with a built-in electric field which occurs atthe heterojunction between appropriately selected materials.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

In the context of organic materials, the terms “donor” and “acceptor”refer to the relative positions of the HOMO and LUMO energy levels oftwo contacting but different organic materials. This is in contrast tothe use of these terms in the inorganic context, where “donor” and“acceptor” may refer to types of dopants that may be used to createinorganic n- and p-types layers, respectively. In the organic context,if the LUMO energy level of one material in contact with another islower, then that material is an acceptor. Otherwise it is a donor. It isenergetically favorable, in the absence of an external bias, forelectrons at a donor-acceptor junction to move into the acceptormaterial, and for holes to move into the donor material.

A significant property in organic semiconductors is carrier mobility.Mobility measures the ease with which a charge carrier can move througha conducting material in response to an electric field. In the contextof organic photosensitive devices, a layer including a material thatconducts preferentially by electrons due to a high electron mobility maybe referred to as an electron transport layer, or ETL. A layer includinga material that conducts preferentially by holes due to a high holemobility may be referred to as a hole transport layer, or HTL.Preferably, but not necessarily, an acceptor material is an ETL and adonor material is a HTL.

Conventional inorganic semiconductor PV cells employ a p-n junction toestablish an internal field. Early organic thin film cells, such asreported by Tang, Appl. Phys Lett. 48, 183 (1986), contain aheterojunction analogous to that employed in a conventional inorganic PVcell. However, it is now recognized that in addition to theestablishment of a p-n type junction, the energy level offset of theheterojunction also plays an important role.

The energy level offset at the organic D-A heterojunction is believed tobe important to the operation of organic PV devices due to thefundamental nature of the photogeneration process in organic materials.Upon optical excitation of an organic material, localized Frenkel orcharge-transfer excitons are generated. For electrical detection orcurrent generation to occur, the bound excitons must be dissociated intotheir constituent electrons and holes. Such a process can be induced bythe built-in electric field, but the efficiency at the electric fieldstypically found in organic devices (F˜10⁶ V/cm) is low. The mostefficient exciton dissociation in organic materials occurs at adonor-acceptor (D-A) interface. At such an interface, the donor materialwith a low ionization potential forms a heterojunction with an acceptormaterial with a high electron affinity. Depending on the alignment ofthe energy levels of the donor and acceptor materials, the dissociationof the exciton can become energetically favorable at such an interface,leading to a free electron polaron in the acceptor material and a freehole polaron in the donor material.

Organic PV cells have many potential advantages when compared totraditional silicon-based devices. Organic PV cells are light weight,economical in materials use, and can be deposited on low costsubstrates, such as flexible plastic foils. However, some organic PVdevices typically have relatively low external quantum efficiency, beingon the order of 1% or less. This is, in part, thought to be due to thesecond order nature of the intrinsic photoconductive process. That is,carrier generation requires exciton generation, diffusion and ionizationor collection. There is an efficiency η associated with each of theseprocesses. Subscripts may be used as follows: P for power efficiency,EXT for external quantum efficiency, A for photon absorption, ED forexciton diffusion, CC for charge collection, and INT for internalquantum efficiency. Using this notation:η_(P)˜η_(EXT)=η_(A)*η_(ED)*η_(CC)η_(EXT)=η_(A)* η_(INT)

The diffusion length (LD) of an exciton is typically much less(L_(D)˜50Å) than the optical absorption length (˜500Å), requiring atrade off between using a thick, and therefore resistive, cell withmultiple or highly folded interfaces, or a thin cell with a low opticalabsorption efficiency.

Typically, when light is absorbed to form an exciton in an organic thinfilm, a singlet exciton is formed. By the mechanism of intersystemcrossing, the singlet exciton may decay to a triplet exciton. In thisprocess energy is lost which will result in a lower efficiency for thedevice. If not for the energy loss from intersystem crossing, it wouldbe desirable to use materials that generate triplet excitons, as tripletexcitons generally have a longer lifetime, and therefore a longerdiffusion length, than do singlet excitons.

SUMMARY OF THE INVENTION

A device is provided having a first electrode, a second electrode, afirst photoactive region having a characteristic absorption wavelengthλ₁ and a second photoactive region having a characteristic absorptionwavelength λ₂. The photoactive regions are disposed between the firstand second electrodes, and further positioned on the same side of areflective layer, such that the first photoactive region is closer tothe reflective layer than the second photoactive region. The materialscomprising the photoactive regions may be selected such that λ₁ is atleast about 10% different from λ₂. The device may further comprise anexciton blocking layer disposed adjacent to and in direct contact withthe organic acceptor material of each photoactive region, wherein theLUMO of each exciton blocking layer other than that closest to thecathode is not more than about 0.3 eV greater than the LUMO of theacceptor material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic PV device comprising an anode, an anodesmoothing layer, a donor layer, an acceptor layer, a blocking layer, anda cathode.

FIG. 2 shows an organic tandem device formed by stacking two cells inseries.

FIG. 3 shows the optical field intensities at λ=450 nm (solid line) andλ=650 nm (dashed line) as a function of the distance from the cathode inthe asymmetric organic tandem cell B (see Table I), the structure ofwhich is schematically shown at the top of FIG. 3.

FIG. 4 shows the calculated external quantum efficiency spectra for thefront (dashed line) and back (solid line) cell of cell B.

FIG. 5 shows the current density vs. voltage (J-V) characteristics ofthe asymmetric organic tandem cell A, in the dark and under variousintensities of simulated AM1.5 G solar illumination.

FIG. 6 shows the illumination intensity (P_(O)) dependence of the powerconversion efficiencies (η_(P)) of asymmetric organic tandem cells (A,filled squares; B, open circles; C, filled triangles) under simulatedAM1.5 G solar illumination, compared with that of the 5% single CuPc/C₆₀hybrid planar-mixed heterojunction cell (open inverted triangles).

FIG. 7 shows the fill factor (FF) of the tandem and single hybrid PM-HJcells shown in FIG. 6.

FIG. 8 shows two possible geometries of a PV device with representativeperpendicular optical path lengths.

FIG. 9 shows absorption spectra of CuPc:C₆₀ films with various mixtureratios, deposited on ITO.

DETAILED DESCRIPTION

An organic photosensitive optoelectronic device is provided. Organicdevices of embodiments of the present invention may be used, forexample, to generate a usable electrical current from incidentelectromagnetic radiation (e.g., PV devices) or may be used to detectincident electromagnetic radiation. Embodiments of the present inventionmay comprise an anode, a cathode, and a photoactive region between theanode and the cathode. The photoactive region is the portion of thephotosensitive device that absorbs electromagnetic radiation to generateexcitons that may dissociate in order to generate an electrical current.Organic photosensitive optoelectronic devices may also include at leastone transparent electrode to allow incident radiation to be absorbed bythe device. Several PV device materials and configurations are describedin U.S. Pat. Nos. 6,657,378, 6,580,027, and 6,352,777, which areincorporated herein by reference in their entirety.

FIG. 1 shows an organic photosensitive optoelectronic device 100. Thefigures are not necessarily drawn to scale. Device 100 may include asubstrate 110, an anode 115, an anode smoothing layer 120, a donor layer125, an acceptor layer 130, a blocking layer 135, and a cathode 140.Cathode 140 may be a compound cathode having a first conductive layerand a second conductive layer. Device 100 may be fabricated bydepositing the layers described, in order. Charge separation may occurpredominantly at the organic heterojunction between donor layer 125 andacceptor layer 130. The built-in potential at the heterojunction isdetermined by the HOMO-LUMO energy level difference between the twomaterials contacting to form the heterojunction. The HOMO-LUMO gapoffset between the donor and acceptor materials produce an electricfield at the donor/acceptor interface that facilitates charge separationfor excitons created within an exciton diffusion length of theinterface.

The specific arrangement of layers illustrated in FIG. 1 is exemplaryonly, and is not intended to be limiting. For example, some of thelayers (such as blocking layers) may be omitted. Other layers (such asreflective layers or additional acceptor and donor layers) may be added.The order of layers may be altered. Arrangements other than thosespecifically described may be used.

The substrate may be any suitable substrate that provides desiredstructural properties. The substrate may be flexible or rigid, planar ornon-planar. The substrate may be transparent, translucent or opaque.Plastic and glass are examples of preferred rigid substrate materials.Plastic and metal foils are examples of preferred flexible substratematerials. The material and thickness of the substrate may be chosen toobtain desired structural and optical properties.

U.S. Pat. No. 6,352,777, incorporated herein by reference, providesexamples of electrodes, or contacts, that may be used in aphotosensitive optoelectronic device. When used herein, the terms“electrode” and “contact” refer to layers that provide a medium fordelivering photo-generated current to an external circuit or providing abias voltage to the device. That is, an electrode, or contact, providesthe interface between the active regions of an organic photosensitiveoptoelectronic device and a wire, lead, trace or other means fortransporting the charge carriers to or from the external circuit. In aphotosensitive optoelectronic device, it is desirable to allow themaximum amount of ambient electromagnetic radiation from the deviceexterior to be admitted to the photoconductively active interior region.That is, the electromagnetic radiation must reach a photoconductivelayer(s), where it can be converted to electricity by photoconductiveabsorption. This often dictates that at least one of the electricalcontacts should be minimally absorbing and minimally reflecting of theincident electromagnetic radiation. That is, such a contact should besubstantially transparent. The opposing electrode may be a reflectivematerial so that light which has passed through the cell without beingabsorbed is reflected back through the cell. As used herein, a layer ofmaterial or a sequence of several layers of different materials is saidto be “transparent” when the layer or layers permit at least 50% of theambient electromagnetic radiation in relevant wavelengths to betransmitted through the layer or layers. Similarly, layers which permitsome, but less that 50% transmission of ambient electromagneticradiation in relevant wavelengths are said to be “semi-transparent.”

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

The electrodes are preferably composed of metals or “metal substitutes”.Herein the term “metal” is used to embrace both materials composed of anelementally pure metal, e.g., Mg, and also metal alloys which arematerials composed of two or more elementally pure metals, e.g., Mg andAg together, denoted Mg:Ag. Here, the term “metal substitute” refers toa material that is not a metal within the normal definition, but whichhas the metal-like properties that are desired in certain appropriateapplications. Commonly used metal substitutes for electrodes and chargetransfer layers would include doped wide-bandgap semiconductors, forexample, transparent conducting oxides such as indium tin oxide (ITO),gallium indium tin oxide (GITO), and zinc indium tin oxide (ZITO). Inparticular, ITO is a highly doped degenerate n+ semiconductor with anoptical bandgap of approximately 3.2 eV, rendering it transparent towavelengths greater than approximately 3900 Å. Another suitable metalsubstitute is the transparent conductive polymer polyanaline (PANI) andits chemical relatives. Metal substitutes may be further selected from awide range of non-metallic materials, wherein the term “non-metallic” ismeant to embrace a wide range of materials provided that the material isfree of metal in its chemically uncombined form. When a metal is presentin its chemically uncombined form, either alone or in combination withone or more other metals as an alloy, the metal may alternatively bereferred to as being present in its metallic form or as being a “freemetal”. Thus, the metal substitute electrodes of the present inventionmay sometimes be referred to as “metal-free” wherein the term“metal-free” is expressly meant to embrace a material free of metal inits chemically uncombined form. Free metals typically have a form ofmetallic bonding that results from a sea of valence electrons which arefree to move in an electronic conduction band throughout the metallattice. While metal substitutes may contain metal constituents they are“non-metallic” on several bases. They are not pure free-metals nor arethey alloys of free-metals. When metals are present in their metallicform, the electronic conduction band tends to provide, among othermetallic properties, a high electrical conductivity as well as a highreflectivity for optical radiation.

Embodiments of the present invention may include, as one or more of thetransparent electrodes of the photosensitive optoelectronic device, ahighly transparent, non-metallic, low resistance cathode such asdisclosed in U.S. Pat. No. 6,420,031, to Parthasarathy et al.(“Parthasarathy '031”), or a highly efficient, low resistancemetallic/non-metallic compound cathode such as disclosed in U.S. Pat.No. 5,703,436 to Forrest et al. (“Forrest '436”), both incorporatedherein by reference in their entirety. Each type of cathode ispreferably prepared in a fabrication process that includes the step ofsputter depositing an ITO layer onto either an organic material, such ascopper phthalocyanine (CuPc), to form a highly transparent,non-metallic, low resistance cathode or onto a thin Mg:Ag layer to forma highly efficient, low resistance metallic/non-metallic compoundcathode.

Herein, the term “cathode” is used in the following manner. In anon-stacked PV device or a single unit of a stacked PV device underambient irradiation and connected with a resistive load and with noexternally applied voltage, e.g., a PV device, electrons move to thecathode from the photo-conducting material. Similarly, the term “anode”is used herein such that in a PV device under illumination, holes moveto the anode from the photo-conducting material, which is equivalent toelectrons moving in the opposite manner. It will be noted that as theterms are used herein, anodes and cathodes may be electrodes or chargetransfer layers.

An organic photosensitive device will comprise at least one photoactiveregion in which light is absorbed to form an excited state, or“exciton”, which may subsequently dissociate in to an electron and ahole. The dissociation of the exciton will typically occur at theheterojunction formed by the juxtaposition of an acceptor layer and adonor layer. For example, in the device of FIG. 1, the “photoactiveregion” may include donor layer 125 and acceptor layer 130.

The acceptor material may be comprised of, for example, perylenes,naphthalenes, fullerenes or nanotubules. An example of an acceptormaterial is 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI).Alternatively, the acceptor layer may be comprised of a fullerenematerial as described in U.S. Pat. No. 6,580,027, incorporated herein byreference in its entirety. Adjacent to the acceptor layer, is a layer oforganic donor-type material. The boundary of the acceptor layer and thedonor layer forms the heterojunction which may produce an internallygenerated electric field. The material for the donor layer may be apthalocyanine or a porphyrin, or a derivative or transition metalcomplex thereof, such as copper pthalocyanine (CuPc). Other suitableacceptor and donor materials may be used.

Through the use of an organometallic material in the photoactive region,devices incorporating such materials may efficiently utilize tripletexcitons. It is believed that the singlet-triplet mixing may be sostrong for organometallic compounds, that the absorptions involveexcitation from the singlet ground states directly to the tripletexcited states, eliminating the losses associated with conversion fromthe singlet excited state to the triplet excited state. The longerlifetime and diffusion length of triplet excitons in comparison tosinglet excitons may allow for the use of a thicker photoactive region,as the triplet excitons may diffuse a greater distance to reach thedonor-acceptor heterojunction, without sacrificing device efficiency.Materials other than organometallics may also be used.

In a preferred embodiment of the invention, the stacked organic layersinclude one or more exciton blocking layers (EBLs) as described in U.S.Pat. No. 6,097,147, Peumans et al, Applied Physics Letters 2000, 76,2650-52, and co-pending application Ser. No. 09/449,801, filed Nov. 26,1999, both incorporated herein by reference. Higher internal andexternal quantum efficiencies have been achieved by the inclusion of anEBL to confine photogenerated excitons to the region near thedissociating interface and to prevent parasitic exciton quenching at aphotosensitive organic/electrode interface. In addition to limiting thevolume over which excitons may diffuse, an EBL can also act as adiffusion barrier to substances introduced during deposition of theelectrodes. In some circumstances, an EBL can be made thick enough tofill pinholes or shorting defects which could otherwise render anorganic PV device non-functional. An EBL can therefore help protectfragile organic layers from damage produced when electrodes aredeposited onto the organic materials.

It is believed that the EBLs derive their exciton blocking property fromhaving a LUMO-HOMO energy gap substantially larger than that of theadjacent organic semiconductor from which excitons are being blocked.Thus, the confined excitons are prohibited from existing in the EBL dueto energy considerations. While it is desirable for the EBL to blockexcitons, it is not desirable for the EBL to block all charge. However,due to the nature of the adjacent energy levels, an EBL may block onesign of charge carrier. By design, an EBL will exist between two otherlayers, usually an organic photosensitive semiconductor layer and aelectrode or charge transfer layer. The adjacent electrode or chargetransfer layer will be in context either a cathode or an anode.Therefore, the material for an EBL in a given position in a device willbe chosen so that the desired sign of carrier will not be impeded in itstransport to the electrode or charge transfer layer. Proper energy levelalignment ensures that no barrier to charge transport exists, preventingan increase in series resistance. For example, it is desirable for amaterial used as a cathode side EBL to have a LUMO energy level closelymatching the LUMO energy level of the adjacent ETL material so that anyundesired barrier to electrons is minimized.

It should be appreciated that the exciton blocking nature of a materialis not an intrinsic property of its HOMO-LUMO energy gap. Whether agiven material will act as an exciton blocker depends upon the relativeHOMO and LUMO energy levels of the adjacent organic photosensitivematerial. Therefore, it is not possible to identify a class of compoundsin isolation as exciton blockers without regard to the device context inwhich they may be used. However, with the teachings herein one ofordinary skill in the art may identify whether a given material willfunction as an exciton blocking layer when used with a selected set ofmaterials to construct an organic PV device.

In a preferred embodiment of the invention, an EBL is situated betweenthe acceptor layer and the cathode. A preferred material for the EBLcomprises 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also calledbathocuproine or BCP), which is believed to have a LUMO-HOMO energylevel separation of about 3.5 eV, orbis(2-methyl-8-hydroxyquinolinoato)-aluminum(III)phenolate (Alq₂OPH).BCP is an effective exciton blocker which can easily transport electronsto the cathode from an acceptor layer.

The EBL layer may be doped with a suitable dopant, including but notlimited to 3,4,9,10-perylenetracarboxylic dianhydride (PTCDA),3,4,9,10-perylenetracarboxylic diimide (PTCDI),3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI),1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), and derivativesthereof. It is thought that the BCP as deposited in the present devicesis amorphous. The present apparently amorphous BCP exciton blockinglayers may exhibit film recrystallization, which is especially rapidunder high light intensities. The resulting morphology change topolycrystalline material results in a lower quality film with possibledefects such as shorts, voids or intrusion of electrode material.Accordingly, it has been found that doping of some EBL materials, suchas BCP, that exhibit this effect with a suitable, relatively large andstable molecule can stabilize the EBL structure to prevent performancedegrading morphology changes. It should be further appreciated thatdoping of an EBL which is transporting electrons in a giving device witha material having a LUMO energy level close to that of the EBL will helpinsure that electron traps are not formed which might produce spacecharge build-up and reduce performance. Additionally, it should beappreciated that relatively low doping densities should minimize excitongeneration at isolated dopant sites. Since such excitons are effectivelyprohibited from diffusing by the surrounding EBL material, suchabsorptions reduce device photoconversion efficiency.

Representative embodiments may also comprise transparent charge transferlayers or charge recombination layers. As described herein chargetransfer layers are distinguished from acceptor and donor layers by thefact that charge transfer layers are frequently, but not necessarily,inorganic (often metals) and they may be chosen not to bephotoconductively active. The term “charge transfer layer” is usedherein to refer to layers similar to but different from electrodes inthat a charge transfer layer only delivers charge carriers from onesubsection of an optoelectronic device to the adjacent subsection. Theterm “charge recombination layer” is used herein to refer to layerssimilar to but different from electrodes in that a charge recombinationlayer allows for the recombination of electrons and holes between tandemphotosensitive devices and may also enhance internal optical fieldstrength near one or more active layers. A charge recombination layercan be constructed of semi-transparent metal nanoclusters, nanoparticleor nanorods as described in U.S. Pat. No. 6,657,378, incorporated hereinby reference in its entirety.

In a preferred embodiment of the invention, an anode-smoothing layer issituated between the anode and the donor layer. A preferred material forthis layer comprises a film of3,4-polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS). Theintroduction of the PEDOT:PSS layer between the anode (ITO) and thedonor layer (CuPc) may lead to greatly improved fabrication yields. Thisis attributed to the ability of the spin-coated PEDOT:PSS film toplanarize the ITO, whose rough surface could otherwise result in shortsthrough the thin molecular layers.

In a further embodiment on the invention, one or more of the layers maybe treated with plasma prior to depositing the next layer. The layersmay be treated, for example, with a mild argon or oxygen plasma. Thistreatment is beneficial as it reduces the series resistance. It isparticularly advantageous that the PEDOT:PSS layer be subject to a mildplasma treatment prior to deposition of the next layer.

The simple layered structure illustrated in FIG. 1 is provided by way ofnon-limiting example, and it is understood that embodiments of theinvention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional devices may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. Organic layers that are not a partof the photoactive region, i.e., organic layers that generally do notabsorb photons that make a significant contribution to photocurrent, maybe referred to as “non-photoactive layers.” Examples of non-photoactivelayers include EBLs and anode-smoothing layers. Other types ofnon-photoactive layers may also be used.

Preferred organic materials for use in the photoactive layers of aphotosensitive device include cyclometallated organometallic compounds.The term “organometallic” as used herein is as generally understood byone of ordinary skill in the art and as given, for example, in“Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A.Tarr, Prentice Hall (1998). Thus, the term organometallic refers tocompounds which have an organic group bonded to a metal through acarbon-metal bond. This class does not include per se coordinationcompounds, which are substances having only donor bonds fromheteroatoms, such as metal complexes of amines, halides, pseudohalides(CN, etc.), and the like. In practice organometallic compounds generallycomprise, in addition to one or more carbon-metal bonds to an organicspecies, one or more donor bonds from a heteroatom. The carbon-metalbond to an organic species refers to a direct bond between a metal and acarbon atom of an organic group, such as phenyl, alkyl, alkenyl, etc.,but does not refer to a metal bond to an “inorganic carbon,” such as thecarbon of CN or CO. The term cyclometallated refers to compounds thatcomprise an bidentate organometallic ligand so that, upon bonding to ametal, a ring structure is formed that includes the metal as one of thering members.

Organic layers may be fabricated using vacuum deposition, spin coating,organic vapor-phase deposition, inkjet printing and other methods knownin the art.

Organic photosensitive optoelectronic devices may function as a PV,photodetector or photoconductor. Whenever the organic photosensitiveoptoelectronic devices of the present invention function as a PV device,the materials used in the photoconductive organic layers and thethicknesses thereof may be selected, for example, to optimize theexternal quantum efficiency of the device. Whenever the organicphotosensitive optoelectronic devices of the present invention functionas photodetectors or photoconductors, the materials used in thephotoconductive organic layers and the thicknesses thereof may beselected, for example, to maximize the sensitivity of the device todesired spectral regions.

This result may be achieved by considering several guidelines that maybe used in the selection of layer thicknesses. It is desirable for theexciton diffusion length, L_(D), to be greater than or comparable to thelayer thickness, L, since it is believed that most exciton dissociationwill occur at an interface. If L_(D) is less than L, then many excitonsmay recombine before dissociation. It is further desirable for the totalphotoconductive layer thickness to be of the order of theelectromagnetic radiation absorption length, 1/α (where α is theabsorption coefficient), so that nearly all of the radiation incident onthe PV device is absorbed to produce excitons. Furthermore, thephotoconductive layer thickness should be as thin as possible to avoidexcess series resistance due to the high bulk resistivity of organicsemiconductors.

Accordingly, these competing guidelines inherently require tradeoffs tobe made in selecting the thickness of the photoconductive organic layersof a photosensitive optoelectronic cell. Thus, on the one hand, athickness that is comparable or larger than the absorption length isdesirable (for a single cell device) in order to absorb the maximumamount of incident radiation. On the other hand, as the photoconductivelayer thickness increases, two undesirable effects are increased. One isthat due to the high series resistance of organic semiconductors, anincreased organic layer thickness increases device resistance andreduces efficiency. Another undesirable effect is that increasing thephotoconductive layer thickness increases the likelihood that excitonswill be generated far from the effective field at a charge-separatinginterface, resulting in enhanced probability of geminate recombinationand, again, reduced efficiency. Therefore, a device configuration isdesirable which balances between these competing effects in a mannerthat produces a high external quantum efficiency for the overall device.

The organic photosensitive optoelectronic devices of the presentinvention may function as photodetectors. In this embodiment, the devicemay be a multilayer organic device, for example as described in U.S.application Ser. No. 10/723,953, filed Nov. 26, 2003, incorporatedherein by reference in its entirety. In this case an external electricfield may be generally applied to facilitate extraction of the separatedcharges.

A concentrator or trapping configuration can be employed to increase theefficiency of the organic photosensitive optoelectronic device, wherephotons are forced to make multiple passes through the thin absorbingregions. U.S. Pat. Nos. 6,333,458 and 6,440,769, incorporated herein byreference in their entirety, addresses this issue by using structuraldesigns that enhance the photoconversion efficiency of photosensitiveoptoelectronic devices by optimizing the optical geometry for highabsorption and for use with optical concentrators that increasecollection efficiency. Such geometries for photosensitive devicessubstantially increase the optical path through the material by trappingthe incident radiation within a reflective cavity or waveguidingstructure, and thereby recycling light by multiple reflection throughthe photoresponsive material. The geometries disclosed in U.S. Pat. Nos.6,333,458 and 6,440,769 therefore enhance the external quantumefficiency of the devices without causing substantial increase in bulkresistance. Included in the geometry of such devices is a firstreflective layer; a transparent insulating layer which should be longerthan the optical coherence length of the incident light in alldimensions to prevent optical microcavity interference effects; atransparent first electrode layer adjacent the transparent insulatinglayer; a photosensitive heterostructure adjacent the transparentelectrode; and a second electrode which is also reflective.

Coatings may be used to focus optical energy into desired regions of adevice. U.S. patent application Ser. No. 10/857,747, which isincorporated by reference in its entirety, provides examples of such acoating.

The power conversion efficiency (η_(P)) of organic cells may be improvedthrough the use of new materials and the introduction of novel devicestructures. The efficiency of organic cells may be improved by using theacceptor material C₆₀ with a long exciton diffusion length (L_(D)≈400Å),or by forming a bulk heterojunction structure, where an interpenetratingnetwork of donor and acceptor materials enhances the probability ofexcitons diffusing to a nearby, “local” D-A interface. One embodiment ofthe present invention provides a copper phthalocyanine (CuPc)/C₆₀organic cell incorporating a hybrid planar-mixed heterojunction (PM-HJ),consisting of a mixed D-A layer sandwiched between homogeneous donor andacceptor layers. The device demonstrates η_(P)=5% under simulated AM1.5G solar illumination.

Stacking two or more cells in series is one way to harvest more photonswhile increasing the open-circuit voltage (V_(OC)) of the cell. A morethan doubling of individual CuPc/PTCBI cell efficiencies of η_(P)=1% toη_(P)=2.5% has been demonstrated, as described in A. Yakimov and S. R.Forrest, Appl. Phys. Lett. 80, 1667 (2002), by stacking two thin cellsin series, with Ag nanoclusters between the subcells providing bothoptical field enhancement and efficient recombination sites for thephotogenerated charges. The photovoltage of this “tandem” cell may betwice that of each individual cell (or subcell). One embodiment of thepresent invention comprises two CuPc/C₆₀ hybrid PM-HJ cells in series,with each cell having a different ratio of CuPc to C₆₀. Thisconfiguration results in η_(P)=(5.7±0.3)% under 1 sun=100 mW/cm²simulated AM1.5 G solar illumination, representing ˜15% increase from asingle hybrid PM-HJ cell. Also, V_(OC) of the tandem cell is greaterthan that of a single PV cell, reaching up to about 1.2 V under highintensity illumination. One embodiment of the present invention employsthe highly efficient material combination of CuPc and C₆₀ in hybridplanar-mixed double heterojunction structures. Without includingantireflection coatings on the substrates, organic PV cells with solarpower conversion efficiencies of 6.5% may be possible using tandemstructures of this type.

A two-subcell CuPc/PTCBI tandem cell has symmetric spectral responsefrom each of the two subcells. The optical interference between theincident light and that reflected from the metal cathode leads to amaximum optical intensity at a perpendicular optical path length of λ/4from the organic/cathode interface, where λ is the incident lightwavelength. As used herein, “perpendicular optical path length” refersto distance/n, measured normal to the surface of the device andintegrated over the path traveled by light, where n is the index ofrefraction of the material and may vary within the material. Hence, an“asymmetric” tandem cell with a front cell rich in long-wavelengthabsorbing molecules, and a back cell rich in short-wavelength absorbingmolecules, may absorb more incident light than an otherwise equivalenttandem cell having equal mixtures of CuPc and C₆₀ in each subcell. Forexample, if CuPc absorbs between λ=550 nm and 750 nm, and C₆₀ betweenλ=350 nm and 550 nm, an asymmetric CuPc/C₆₀ hybrid PM-HJ tandem cell mayinclude a front cell with a thicker homogeneous CuPc layer and a thinnerC₆₀ layer than the back cell. A tradeoff between the homogeneous andmixed layer thicknesses may also be used to balance the photocurrent inthe two subcells, due to the short exciton diffusion lengths in thehomogeneous layers and the low charge mobilities in the mixed layers.

The efficiency of a CuPc/C₆₀ hybrid PM-HJ tandem cell may be maiximizedby modeling the current density vs. voltage (J-V) characteristics ofsubcell i (i=1,2 denoting the front and back cell, respectively)following: $\begin{matrix}\begin{matrix}{{J_{i}( V_{i} )} = {{J_{d,i}( V_{i} )} + {J_{{Ph},i}( V_{i} )}}} \\{{= {{J_{s,i}\{ {{\exp\lbrack \frac{q( {V_{i} - {J_{i}R_{S,i}}} )}{n_{i}{kT}} \rbrack} - 1} \}} + {J_{{Ph},i}^{0}{\eta_{{CC},i}( V_{i} )}}}},}\end{matrix} & (1)\end{matrix}$where J_(d,i) and J_(Ph,i) are the dark and photo current densities,respectively, J_(s,i) is the reverse-bias saturation current, n_(i) isthe ideality factor, R_(S,i) is the cell series resistance, q is theelectron charge, k is Boltzmann's constant, and T is the temperature.Using a model that considers both optical interference and excitondiffusion, the photocurrent density J_(Ph,i) ⁰ under an incident opticalpower density P_(O) may be obtained, assuming all photogenerated chargesare collected at the electrodes. This assumption may not hold for a cellwith a mixed layer, where charge carrier mobilities are significantlyreduced from those in homogeneous layers due to molecular intermixing,leading to recombination of photogenerated charges within the mixedlayer. The charge collection efficiency η_(CC), or the proportion ofcharges collected at the electrodes, as a function of the appliedvoltage V and the mixed layer thickness d_(m) is $\begin{matrix}{{{\eta_{CC}(V)} = {\frac{L_{c}(V)}{d_{m}}\{ {1 - {\exp\lbrack {- \frac{d_{m}}{L_{c}(V)}} \rbrack}} \}}},} & (2)\end{matrix}$where L_(c)(V)=L_(c0)(V_(bi)×V)/V is the charge collection length,L_(c0) is a constant, and V_(bi) is the built-in potential. GivenJ_(i)=J_(i)(V_(i)) (i=1,2), the J-V characteristics of the tandem cellare obtained from the requirement that J=J₁=J₂, and V=V₁+V₂, from whichthe PV cell performance parameters (short-circuit current densityJ_(SC), open-circuit voltage V_(OC), fill factor FF, and powerconversion efficiency η_(P)) are obtained.

Table 1 provides the device structures of three tandem cells. Table 2summarizes the parameter values used in the model. Referring to Table 1,cell A has mixed layer thicknesses based on given asymmetric homogeneouslayer thicknesses, leading to η_(P)=5.2% under 1 sun AM1.5 G solarillumination. The combination of photoactive layer thicknesses in cell Bleads to a higher efficiency of η_(P)=5.9%. A PTCBI layer in the frontcell may also contribute to the photocurrent when the front-cellhomogeneous C₆₀ layer is removed, so that CuPc molecules in the mixedlayer may form an exciton dissociation interface with PTCBI. This leadsto a higher J_(SC) and a maximum η_(P)=6.5% in cell C, as the PTCBIabsorption fills the gap between the absorption regions of CuPc and C₆₀,at around λ=550 nm. TABLE 1 Cell Front cell Back cell J_(SC) V_(OC)η_(P) label CuPc CuPc:C₆₀ C₆₀ PTCBI CuPc CuPc:C₆₀ C₆₀ BCP (mA/cm²) (V)FF (%) A 100 180 20 50 20 130 250 75 9.4 1.03 0.54 5.2 B 75 125 80 50 60130 160 75 9.7 1.03 0.59 5.9 C 90 110 0 50 50 100 210 100 10.3 1.04 0.616.5

TABLE 2 J_(s) R_(S) L_(c0) V_(bi) L_(D) (Å) (nA/cm²) (Ω · cm²) n (Å) (V)CuPc C₆₀ PTCBI 35 0.25 1.6 400 0.65 80 400 30

In one embodiment of the invention, photoactive regions are disposedbetween two electrodes. In preferred embodiments of the invention, thephotoactive regions comprise hybrid planar-mixed heterojunction (PM-HJ)devices, as described in U.S. patent application Ser. No. 10/822774,incorporated herein by reference in its entirety.

FIG. 2 shows an organic photoactive device 200 according to oneembodiment of the invention. Device 200 may comprise a substrate 210 onwhich is deposited a first electrode 220, a first (or “front”) organicphotoactive region 230, an intervening layer 240, a second (or “back”)photoactive region 250, and a second electrode 260. The organicphotoactive regions 230 and 250 comprise an organic acceptor materialand an organic donor material. In preferred embodiments of theinvention, the first photoactive region 230 further comprises a firstorganic layer 231 comprising an unmixed organic acceptor material; asecond organic layer 232 comprising a mixture of the organic acceptormaterial of unmixed organic acceptor layer 231 and an organic donormaterial; a third organic layer 233, comprising the unmixed donormaterial of second organic layer 232; and an exciton blocking layer 234.In other preferred embodiments, acceptor layers 231 and 251 or donorlayers 233 and 253 may be absent. In another preferred embodiment of theinvention, the second photoactive region 250 comprises a similararrangement of organic materials as first photoactive region 230. In apreferred embodiment of the invention, intervening layer 240 comprises acharge recombination zone. In another preferred embodiment, interveninglayer 240 may comprise one or more electrodes, where multiple electrodesmay be separated by an insulating layer.

In another embodiment of the present invention, the mixture of theorganic acceptor material and the organic donor material in a mixedorganic layer, such as organic layer 232 may occur in a ratio rangingfrom about 10:1 to about 1:10 by weight, respectively. In oneembodiment, an organic layer including a mixture of acceptor and donormaterials (such as organic layer 232), and an organic layer thatincludes only an acceptor material or a donor material (such as secondorganic layer 231 or 233) may be present.

When an EBL is deposited in a layer adjacent to and in direct contactwith a deposited cathode, the EBL may be damaged. This damage isbelieved to be advantageous in that it may allow charge carriers to passacross the EBL more easily, while still preventing excitons from doingso. It is believed that selecting materials for the EBL and organicacceptor layers such that the LUMO of each EBL is not more than about0.3 eV greater than the LUMO of the adjacent acceptor material producesa similar result. To obtain favorable charge transport properties, it istherefore preferable that an EBL disposed adjacent to an acceptor layeris (1) not separated from the second electrode by a photoactive region;and/or (2) has a LUMO not more than about 0.3 eV greater than the LUMOof an adjacent photoactive region. If a particular EBL is not separatedfrom the second electrode by a photoactive region, such that the EBL issubject to damage during deposition of the second electrode, the LUMOdifference between the EBL and the acceptor is less important and thecriteria used to select the EBL material may be weighted more towardsfactors other than LUMO.

In a preferred embodiment of the device, the second exciton blockinglayer 254 comprises a different material than the first exciton blockinglayer 234. Because exciton blocking layer 254 is not separated from thesecond electrode by a photoactive region, a broader selection ofmaterials may be available. The material of exciton blocking material254 may have a LUMO of not more than about 0.3 eV greater than the LUMOof organic acceptor layer 253, or it may have a LUMO that is greater, btcharge transport may still be favorable due to damage caused by thedeposition of the second electrode 260. Preferred materials for excitonblocking layer 254 include BCP, and preferred materials for excitonblocking layer 234 include PTCBI.

In another embodiment of the invention, organic cell 200 furthercomprises an intervening layer 240. Intervening layer 240 may comprise acharge recombination zone. In a preferred embodiment of the invention,the charge recombination zone comprises a p-doped organic material, suchas m-MTDATA:F₄-TCNQ or BTQBT:PTCDA, and the charge recombination zonefurther comprises nanoparticles 241. It is especially preferred that thenanoparticles comprise Ag or another metal or metallic material. Othermaterials may be used.

In a tandem cell, it may be advantageous to use different acceptor anddonor materials, or the same acceptor and donor materials in differentratios, in each subcell. Using different materials or different ratiosof the same materials may allow the cell to absorb light in a greaterrange of wavelengths than if the same materials in the same ratios areused in each subcell. In a preferred embodiment of the invention,organic regions 230 and 250 comprise different acceptor and donormaterials. Organic regions 230 and 250 may also comprise the sameacceptor and donor materials, wherein the mixed organic layers 232 and252 comprise different ratios of the acceptor and donor materials. Theorganic acceptor material of organic regions 230 and 250 may be C₆₀. Theorganic donor material of photoactive regions 230 and 250 may be CuPc.Other suitable organic donor materials include lead phthalocyanine(PbPc), metal containing porphyrins, metal-free porphyrins, rubrene,metal containing phthalocyanines, metal-free phthalocyanines, diamines(such as NPD), and functionalized variants thereof, includingnaphthalocyanines. Other suitable organic acceptor materials includePTCBI, C₇₀, fullerenes, perylenes, catacondensed conjugated molecularsystems such as linear polyacenes (including anthracene, napthalene,tetracene, and pentacene), pyrene, coronene, and functionalized variantsthereof. This listing is not meant to be comprehensive, and othersuitable acceptor and donor materials may be used.

In an especially preferred embodiment of the present invention, theanode comprises a transparent, conducting indium-tin-oxide (ITO) layeron a glass substrate, and the cathode comprises a 1000 Å thick thermallyevaporated Ag electrode. The photoactive region of each subcellcomprises a hybrid PM-HJ, i.e., a mixed CuPc:C₆₀ layer disposed betweenhomogeneous CuPc and C₆₀ layers, which combines the advantages of aplanar HJ between homogeneous layers (good transport of photogeneratedcharge carriers to their respective electrodes) and a mixed layer (highexciton diffusion efficiency). A thin layer of3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI) andbathocuproine (BCP) is used as the exciton-blocking layer (EBL) in thefront (nearest the ITO) and back (nearest the cathode) subcells,respectively, thereby forming a high efficiency double heterojunction PVstructure. A charge recombination zone for electrons generated in thefront cell and holes generated in the back cell is disposed between thesubcells. The recombination centers comprise Ag nanoclusters depositedin an ultrathin (˜5Å, average thickness) layer disposed in a 50 Å thick4,4′,4″-tris(3 -methyl-phenyl-phenyl-amino)triphenylamine (m-MTDATA)p-doped with 5 mol % tetrafluoro-tetracyano-quinodimethane (F₄-TCNQ).Device fabrication procedures and methods of characterization may bethose known in the art.

It is understood that the embodiments described herein are exemplaryonly, and that other embodiments may be used in accordance with thepresent invention. For example, the order of the layers illustrated maybe altered. For example, the positions of the organic layers 230 and 250may be switched, with appropriate repositioning of blocking layers, etc.Additional layers may or may not also be present, such as blockinglayers, charge recombination layers, etc. For example, blocking layersmay be removed, and/or additional blocking layers may be present.Non-organic regions may be present, and may be used to adjust theposition of the organic regions relative to a reflecting layer.Different materials than those specifically described may be used. Forexample, a device where all of the electrodes are ITO may be fabricatedsuch that the device may be transparent to some degree. Additionally,the device could be fabricated onto a substrate, and then applied to asupporting surface, such that the last electrode deposited is closest tothe supporting surface. Acceptor and donor layers may not be present.For example, donor or acceptor layers 231, 251, 233, and 253 may beabsent. Although many embodiments are described with respect to solarcells, other embodiments may be used in other types of devices, such asa photodetector.

Where a layer is described as an “unmixed” acceptor or donor layer, the“unmixed” layer may include very small amounts of the opposite materialas an impurity. A material may be considered an impurity if theconcentration is significantly lower than the amount needed forpercolation in the layer, i.e., less than about 5% by weight.Preferably, any impurity is present in a much lower amount, such as lessthan 1% by weight or most preferably less than about 0.1% by weight.Depending upon the processes and process parameters used to fabricatedevices, some impurities of the materials in immediately adjacent layersmay be unavoidable.

Organic materials may have absorption spectra with maxima at specificwavelengths. As used herein, the term “characteristic absorptionwavelength” refers to the wavelength at which the absorption spectrum ofa material has a maximum.

Device 200 may include at least one reflective layer. In one embodimentof the invention, second electrode 260 is a reflective layer. Otherconfigurations may be used, such as the use of a separate reflectivelayer, or a top emitting (or absorbing) device where the substrate orthe first electrode is the reflective layer. A “reflective” layer may bea metal layer, or another type of layer that reflects, such as anaperiodic or periodic dieliectric stack. The use of a reflective layerleads to an optical field strength that varies with wavelength and withposition in a direction perpendicular to the reflective layer. For anygiven wavelength, there are maxima in the optical field strength as afunction of position. See, for example, FIG. 3. For photosensitivedevices, it is desirable to locate a photoactive region having aparticular characteristic absorption wavelength such that the positionhaving a maximum as a function of position for that wavelength islocated within or near the photoactive region. For a photoactive regionhaving multiple materials, the characteristic absorption wavelength isbased on the maxima of the absorption spectra of the entire region.“Near” may mean, for example, at a distance that is not more than about0.05 λn from the photoactive region in question, where n is the index ofrefraction of the material in which the maximum occurs. Preferably, themaximum is disposed within the photoactive region. Locating thephotoactive region in this manner will lead to enhanced absorption. Forsome applications, it is desirable to use light absorbing regions thathave significantly different characteristic absorption wavelengths. Sucha difference may allow for absorption of a broader range of wavelengths.In one embodiment of the invention, organic regions 250 and 230 havecharacteristic absorption wavelengths at λ₁ and λ₂, respectively. It ispreferred that λ₁ be at least about 10% different from λ₂.Characteristic absorption wavelength is only one way of quantifying“different” absorption spectra. Another way of quantifying differentabsorption spectra is that the wavelength of at least one of the topthree absorption peaks of one photoactive region is at least 10%different from all of the wavelengths of the top 3 absorption peaks ofanother photoactive region. Yet another way to quatify differentabsorption spectra is to superimpose two normalized spectra on eachother, and to measure the area that overlaps. It this overlap area is80% or less of the total area of one of the spectra, the spectra can beconsidered significantly different. For example, two materials may havesimilar characteristic absorption wavelengths, but other features (suchas subpeaks) that are significantly different, and possiblycomplementary for purposes of absorbing a broad spectra of incidentlight. Such an embodiment is intended to be within the scope of certainaspects of the invention.

Many photoactive materials (and combinations of materials, forphotoactive regions having multiple materials) may have a plurality ofabsorption peaks. A photoactive region that strongly absorbs aparticular wavelength of light may be placed at a position where theoptical field strength for that wavelength is strong. In one embodiment,a local peak in the absorption spectra of a photoactive region is usedto determine a favorable position for the photoactive region. Thephotoactive region may be located at or near a maximum in the opticalfield intensity for the wavelength for which the photoactive region hasa local maximum. For devices intended to absorb the solar spectrum,wavelengths between 350 and 1300 nm may be of greater importance. Ingeneral terms, it is preferable to increase or maximixe the overlap ofoptical field intensity at a particular wavelength or range ofwavelengths with a photoactive region that is a strong absorber of thosewavelengths. One way to achieve this is adjusting the position of aphotoactive region towards a position where there is a greater overlapbetween the absorption of the photoactive region and the optical fieldintensity (both as functions of wavelength). Another way is to alter theabsorption characteristics of a photoactive region by altering thematerials therein, or the ratio of materials, to achieve a greateroverlap between the absorption spectrum and the optical field intensity(both as functions of wavelength) at the position of the photoactiveregion.

One way to describe such a match is to determine the wavelength of thetop 3 absorption peaks for a photoactive region, and place thephotoactive region such that a peak in the optical field strength forone of these three wavelengths is either in the photoactive region orwithin 0.05 λ/n of the photoactive region, where λ is the wavelength ofthe peak that matches the maximum in the optical field strength, and nis the index of refraction of the layer in which the peak in the opticalfield intensity is located. Another way to describe such a match is toconsider the wavelengths of all of the absorption peaks of thephotoactive region(s). The “wavelength” of an absorption peak is thelocal maximum of the absorption spectra for the peak, and the “topthree” peaks are the peaks having the three highest local maximums. Whendetermining the “top” wavelength or wavelengths, the wavelength rangemay be limited in some embodiments. For example, for some devicesintended to absorb the solar spectrum, the range of wavelengthconsidered may be limited to 350-1300 nm because a large fraction of theusable energy of the solar spectra falls within this range, althoughbroader ranges may also be used in some embodiments, includingembodiments intended to absorb the solar spectrum.

Locating photoactive regions as described may lead to an increase in theamount of incident light absorbed. In preferred embodiments of theinvention, materials and positions of the photoactive regions areselected such that at least about 10%, and more preferably at leastabout 20%, of the total incident electric field intensity is located ina photoactive region having absorption characteristics such that theenergy may be absorbed. As used herein, “optical field intensity” refersto the integral of the square of the electric field over a region. Thus,the total incident electric field intensity is the integral of thesquare of the electric field over the entire device, and the totalelectric field of the photoactive regions is the sum of the electricfield integrated over each of the photoactive regions. Thus, for aregion R, the intensity I_(R) will be calculated as:I _(R)=∫_(R) |{right arrow over (E)}| ²as will be appreciated by one skilled in the art. Moreover, the opticalfield intensity at each point is also a function of wavelength. It ispreferred to increase the integral as a function of position over thephotoactive regions of the device of: the integral over wavelength ofthe product of the absorption characteristic of the photoactive region(which may be a function of position and wavelength) with the opticalfield intensity (which may also be a function of both position andwavelength). This quantity, divided by the total optical fieldintensity, is the percentage of optical field intensity that may beabsorbed by the device, and which is preferably at least 10%, and morepreferably at least 20%. The percentage of optical field intensity thatmay be absorbed may be increased, for example, by selecting materialsthat are good absorbers of particular wavelengths of light, and placingthem where the optical field intensity for that particular wavelength isgreat. It is believed that this will lead to increased absorption by thephotoactive regions, and thus an improved efficiency of the device. In apreferred embodiment, the optical field intensity is based upon a solarspectrum. Note that peak matching may not be the only way to achieve 10%or 20% as described above. Matching photoactive regions that have strongabsorption at a particular wavelength (regardless of whether there is apeak) with a strong value for that wavelength in the optical fieldintensity is one such way to accomplish this goal. By calculating theintegral described above, it is possible to determine whether a devicewill have strong absorption or not.

For the case of a single reflective layer, which is a good approximationof many embodiments, there is a maximum in the optical field strengthfor a particular wavelength λ at an optical path length λ/4 distant fromthe reflective layer. Hence, it is further preferred that at least aportion of the first photoactive region 250 is disposed at aperpendicular optical path length about λ₁/4±25% from the edge of thereflective layer closest to the first photoactive region, and at least aportion of the second photoactive region 230 is disposed at aperpendicular optical path length about λ₂/4±25% from the edge of thereflective layer closest to the second photoactive region, where λ₁ andλ₂ are the wavelengths at which the first and second photoactive regionsare strong absorbers. A “strong absorber” may be quantified in a numberof ways. In one embodiment, the wavelength of at least one of theabsorption peaks of the second photoactive region may be greater thanthe wavelength of at least one of the absorption peaks of the firstphotoactive region. In another embodiment, the wavelength of at leastone of the top three absorption peaks of the second photoactive regionmay be greater than the wavelength of at least one of the top threeabsorption peaks of the first photoactive region. The 25% margin is ameasure of how far the wavelength of the absorption peak may be from themaximum in the optical field strength, while still maintaining asignificant overlap between a strong optical field strength and a strongabsorption for that wavelength and nearby avelengths. More generally,for the case of a single reflective layer and configurations that have asimilar optical field intensity profile, it is preferred to place longerwavelength absorbing materials proportionally farther from thereflecting surface than shorter wavelength absorbing materials, wherethe constant of proportionality is λ/n, where n is the index ofrefraction of the materials in the stack. Where n varies throughout thestack, a spatially weighted mean index of refraction of the materialscomprising the stack may be used. For more complex opticalconfigurations, one of skill in the art, with the benefit of thisdisclosure, will be able to determine the location of maxima in theoptical field strength.

While many embodiments of the invention are described with respect totwo stacked cells, it is understood that a greater number of stackedcells may be used, and the concepts relating to positioning the cellsand the blocking layers used adjacent to the cells are generallyapplicable to stacks having more than two cells.

As used herein, and as would be understood by one skilled in the art,the term “blocking layer” means that the layer provides a barrier thatsignificantly inhibits transport of charge carriers and/or excitonsthrough the device, without suggesting that the layer necessarilycompletely blocks the charge carriers and/or excitons. The presence ofsuch a blocking layer in a device may result in substantially higherefficiencies as compared to a similar device lacking a blocking layer.

FIG. 3 shows the optical field intensities at λ=450 nm (solid line) andλ=650 nm (dashed line) as a function of the distance from the cathode inthe asymmetric organic tandem cell B (see Table I), whose structure isshown schematically at the top of FIG. 3. The intensity at λ=450 nmpeaks at approximately 400 Å from the reflecting Ag cathode, or about300 Å closer than that for λ=650 nm. Hence, making the front cell richin shorter wavelength absorbing material(s) and the back cell rich inlower wavelength absorbing material(s) may lead to increased absorptionof a broad spectrum. In cell B the back cell has a significantly thickerhomogeneous C₆₀ layer than the front cell, leading to a higher externalquantum efficiency in the C₆₀ absorption region (λ<550 nm), as shown inFIG. 4. As the light intensity at λ≈650 nm is primarily located in thefront cell, the quantum efficiency at 550 nm<λ<750 nm may be higher forthe front cell, although the thicknesses of the homogeneous CuPc and themixed layers are approximately the same in both subcells to balancetheir photocurrents.

FIG. 4 shows the calculated external quantum efficiency spectra for thefront (dashed line) and back (solid line) cell of cell B. The asymmetricspectral responses from the two subcells result from the asymmetrictandem cell structure as well as the optical interference.

FIG. 5 shows the current density vs. voltage (J-P) characteristics ofthe asymmetric organic tandem cell A, in the dark and under variousintensities of simulated AM1.5 G solar illumination. The experimentalJ-V characteristics of tandem cell A in the dark and under variousintensities of simulated AM1.5 G solar illumination are shown (opensymbols). A rectification ratio of 10⁵-10⁶ at ±1.5 V is typical. Theopen-circuit voltage is V_(OC)=1.04 V under 1 sun illumination, andapproaches 1.2 V under 10 suns, which may be twice of that of a singleCuPc/C₆₀ hybrid PM-HJ cell. The solid lines are modeled J-Vcharacteristics, which agrees with the experimental data except for thereverse-bias dark current, in which case generation-recombinationcurrent or thermally assisted tunneling may significantly contribute toJ_(d).

FIG. 6 shows the measured illumination intensity (P_(O)) dependence ofthe power conversion efficiencies (η_(P)) of various asymmetric organictandem cells (A, filled squares; B, open circles; C, filled triangles)under simulated AM1.5 G solar illumination, compared with that of a 5%single CuPc/C₆₀ hybrid planar-mixed heterojunction cell (open invertedtriangles). The power conversion efficiency of tandem cell A (filledsquares), derived from the experimental J-V characteristics in FIG. 5,reaches a maximum of η_(P)=(5.4±0.3)% at P_(O)=0.34 suns. Under higherintensity illumination, the FF decreases (see FIG. 7) due to therelatively thick mixed layers. With thinner mixed layers, tandem cell B(open circles) exhibits a high FF=0.56 even under an intenseillumination of approximately 11 suns. This leads to η_(P)=(5.7±0.3)% atP_(O)≧1 sun, in agreement with the model. However, tandem cell C (filledtriangles) has a lower efficiency than model prediction (6.5%), mainlydue to a low FF≈0.51. This may suggest a small energy barrier at theC₆₀/PTCBI interface impeding electrons transporting to the chargerecombination zone. Nevertheless, the efficiencies of tandem cells A andB are higher than the 5% single CuPc/C₆₀ hybrid PM-HJ cell (openinverted triangles in FIG. 6), demonstrating the effectiveness ofstacking cells.

FIG. 7 shows the fill factor (FF) of the tandem and single hybrid PM-HJcells shown in FIG. 6. Under higher intensity illumination, the FFdecreases due to the relatively thick mixed layers. With thinner mixedlayers, tandem cell B (open circles) exhibits a high FF=0.56 even underan intense illumination of approximately 11 suns.

FIG. 8 shows two possible geometries of a PV device 810 and 820, withrepresentative perpendicular optical path lengths 815 and 825. Theperpendicular optical path length is measured normal to the surface ofthe device.

FIG. 9 shows absorption spectra of CuPc:C₆₀ films with various mixtureratios, deposited on ITO. The concentrations of CuPc in mixed films are100% CuPc(CuPc single layer) 910, 62% 920, 40% 930, 33% 940 and 21% 950.The pure CuPc film has two peaks centered at wavelengths of 620 nm and695 nm. The longer wavelength peak is due to molecular Frenkel excitongeneration, whereas the shorter wavelength feature is attributed to theformation of CuPc aggregates. The longer wavelength peak is dominant inthe gas phase or dilute solution. FIG. 9 shows that the magnitude of thelonger wavelength peak increases with increasing C₆₀ content.Accordingly, CuPc molecules show a lower tendency to aggregate withincreasing C₆₀ content. This suggests that an increase in C₆₀concentration inhibits CuPc aggregation, thereby reducing hole transportin the mixed film, perhaps leading to a low carrier collectionefficiency. This is reflected in the reduced power efficiency(η_(P)=(2.6±0.1)%, see Table 2) of a CuPc:C60 (1:2) mixed layer PV cell.However, at a concentration of 1:1, there is sufficient aggregation ofCuPc molecules to allow for low resistance hole transport, while themuch higher symmetry C₆₀ molecules may also form a percolation path forefficient electron transport to the cathode.

Table 1 shows the layer thicknesses (in Å) of three organic tandemphotovoltaic cells as well as predicted performance parameters(short-circuit current density J_(SC), open-circuit voltage V_(OC), fillfactor FF, and power conversion efficiency η_(P)) under 1 sun AM1.5 Gsolar illumination. The charge recombination zone in each tandem cellconsists of a 5 Å thick Ag nanoclusters layer and a 50 Å thick m-MTDATAdoped with 5 mol % F₄-TCNQ.

Table 2 shows the parameters used in modeling the J-V characteristics ofCuPc/C₆₀ hybrid PM-HJ tandem PV cells.

It is understood that the embodiments described herein are exemplaryonly, and that other embodiments may be used in accordance with thepresent invention. For example, the order of the layers illustrated maybe altered. For example, in FIGS. 1 and 2, the positions of thephotoactive layers, i.e., organic regions 230 and 250 may be switched,with appropriate repositioning of blocking layers, etc. Additionallayers may or may not also be present, such as blocking layers, chargerecombination layers, etc. For example, blocking layers may be removed,and/or additional blocking layers may be present. Non-photoactiveregions may be present, and may be used to adjust the position of thephotoactive regions relative to a reflecting layer. Various solar cellconfigurations may be used, such as tandem solar cells. Differentmaterials than those specifically described may be used. For example, adevice where all of the electrodes are ITO may be fabricated such thatthe device may be transparent to some degree. Additionally, the devicecould be fabricated onto a substrate, and then applied to a supportingsurface, such that the last electrode deposited is closest to thesupporting surface. Although many embodiments are described with respectto solar cells, other embodiments may be used in other types ofphotosensitive devices having a D-A heterojunction, such as aphotodetector.

The power efficiencies achieved by embodiments of the invention arehigher than previous efficiencies achieved for organic solar cells.These results may be due to interactions between several features ofembodiment of the invention, including the use of an unmixed organicphotoactive layer in connection with a mixed organic photoactive layer,with thicknesses and positions selected with efficiency in mind.Embodiments of the invention may be capable of reaching power conversionefficiencies approaching that of a-Si cells, currently in production,with efficiencies of 7%-10%. It is expected that with refinement ofdevices consistent with embodiments of the invention, even higher powerefficiencies may be achieved. For example, by applying simpleanti-reflection coatings to the glass substrates, an additional 10%improvement to efficiencies may be possible, suggesting that the tandemcell structure proposed here may attain efficiencies in excess of 7%.Stacking more than two cells in series could help to harvest more light,although an efficient cell structure is more difficult to achieve. Anultimate advantage of the asymmetric tandem cell structure is that itenables the incorporation of different donor-acceptor materialcombinations in the individual subcells to cover a broader solarspectrum region than the current CuPc-C₆₀ system. Provided that highproduction yields and long operational lifetimes are possible inproperly packaged organic solar cell modules, the asymmetric hybridPM-HJ tandem cell has considerable potential for use a variety ofapplications.

EXAMPLES

In one embodiment of the invention, an efficient photovoltaic cell isprovided. A cell with two stacked hybrid planar-mixed heterojunctioncells was fabricated on a glass substrate pre-coated with transparentconducting ITO. The device has the structure: ITO/75 Å CuPc/122 ÅCuPc:C₆₀ (1.2:1 by weight)/80 Å C₆₀/50 Å PTCBI/5 Å Ag/50 Åm-MTDATA:F₄-TCNQ/60 Å CuPc/132 Å CuPc:C₆₀ (1.2:1 by weight)/160 Å C₆₀/75Å BCP/Ag. The cell farther away from the cathode is slightly rich inCuPc, which absorbs in the spectral region from 550 nm to 750 nm,whereas the cell closer to the cathode is rich in C₆₀, which absorbs inthe spectral region from 350 nm to 550 nm. A maximum power efficiencywas measured of (5.6±0.3)% under 1 to 4 suns simulated AM1.5 G solarillumination.

The organic hybrid planar-mixed heterojunction photovoltaic cells werefabricated on glass substrates precoated with a ˜1500 Å thicktransparent, conducting ITO anode with a sheet resistance of 15 Ω/sq.The substrates were cleaned in solvent followed by UV-ozone treatmentfor 5 minutes. The organic layers and a metal cathode were deposited viathermal evaporation in a high vacuum chamber with a base pressure˜2×10⁻⁷ Torr. A CuPc layer with a thickness of d_(D)˜50-200 Å, aco-deposited layer of CuPc:C₆₀ (1:1 by weight) with a thickness ofd_(m)˜0-300 Å, and a C₆₀ layer with a thickness of d_(A)˜250-400 Å, aresequentially deposited onto the ITO anode, followed by a 100 Å thickexciton-blocking layer of BCP. Finally, a 1000 Å thick Ag cathode wasevaporated through a shadow mask with 1 mm diameter openings.

Current-voltage characteristics of the PV cells at 25° C. in the darkand under simulated AM1.5 G solar illumination from a 150 W Xe-arc lamp(Oriel Instruments) were measured using an HP 4155B semiconductorparameter analyzer. The illumination intensity was varied using neutraldensity filters and measured with a calibrated broadband optical powermeter (Oriel Instruments).

Although the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed may therefore include variations from theparticular examples and preferred embodiments described herein, as willbe apparent to one of skill in the art.

1. A device comprising: a first electrode; a second electrode; a firstorganic photoactive region disposed between the first electrode and thesecond electrode; and a second organic photoactive region disposedbetween the first electrode and the second electrode; wherein the firstorganic photoactive region and the second organic photoactive regionhave different absorption characteristics; wherein the device includes areflective layer, and the first organic photoactive region and thesecond organic photoactive region are disposed on the same side of thereflective layer.
 2. The device of claim 1, wherein the device absorbsat least 10% of the total optical field intensity when the device isexposed to incident light having a solar spectra.
 3. The device of claim1, wherein the device absorbs at least 20% of the total optical fieldintensity when the device is exposed to incident light having a solarspectra.
 4. The device of claim 1, wherein the first organic photoactiveregion is disposed closer to the reflective layer than the secondorganic photoactive region, and wherein the wavelength of at least oneof the top three absorption peaks of the second photoactive region isgreater than at least one of the top three absorption peaks of the firstphotoactive region.
 5. The device of claim 4, wherein the wavelength ofat least one of the top three absorption peaks of the second photoactiveregion is at least 10% greater than at least one of the top threeabsorption peaks of the first photoactive region.
 6. The device of claim1, wherein the first organic photoactive region is disposed closer tothe reflective layer than the second organic photoactive region, andwherein the second organic photoactive region is a stronger absorber atlarger wavelengths than the first organic photoactive region.
 7. Thedevice of claim 1, wherein the first electrode is the reflective layer.8. The device of claim 1, wherein at least a portion of the firstorganic photoactive region is disposed at a perpendicular optical pathlength of λ₁/4±25% from the edge of the reflective layer closest to thefirst organic photoactive region, and at least a portion of the secondorganic photoactive region is disposed at a perpendicular optical pathlength of λ₂/4±25% from the edge of the reflective layer closest to thesecond organic photoactive region, where λ₁ is the wavelength of one ofthe top three absorption peaks of the first photoactive region, and λ₂is the wavelength of one of the top three absorption peaks of the secondphotoactive region.
 9. The device of claim 1, wherein each photoactiveregion further comprises: an organic acceptor material; and an organicdonor material in direct contact with the organic acceptor material. 10.The device of claim 9, wherein each organic photoactive region includesthe same organic acceptor material and organic donor material, andwherein the first organic photoactive region and the second organicphotoactive region comprise a different ratio of organic acceptormaterial to organic donor material.
 11. The device of claim 10, whereinthe organic donor material of each organic photoactive region is CuPcand the organic acceptor material of each organic photoactive region isC₆₀.
 12. The device of claim 11, wherein the organic acceptor materialand organic donor material of the first organic photoactive region aredifferent from the organic acceptor material and organic donor materialof the second organic photoactive region.
 13. The device of claim 12,wherein the organic donor material of the first organic photoactiveregion is CuPc, the organic acceptor material of the first organicphotoactive region is C₆₀, the organic donor material of the secondorganic photoactive region is PbPc, and the organic acceptor material ofthe second organic photoactive region is PTCBI.
 14. The device of claim1, wherein each photoactive region further comprises: a first organiclayer comprising a mixture of an organic acceptor material and anorganic donor material; a second organic layer in direct contact withthe first organic layer, wherein the second organic layer comprises anunmixed layer of the organic donor material of the first organic layer;and a third organic layer in direct contact with the first organiclayer, wherein the third organic layer comprises an unmixed layer of theorganic acceptor material of the first organic layer; wherein theexciton blocking layer is disposed adjacent to and in direct physicalcontact with the third organic layer.
 15. The device of claim 1, whereineach photoactive region further comprises: a first organic layercomprising a mixture of an organic acceptor material and an organicdonor material; and a second organic layer in direct contact with thefirst organic layer, wherein the second organic layer comprises anunmixed layer of the organic donor material of the first organic layer;wherein the exciton blocking layer is disposed adjacent to and in directphysical contact with the first organic layer.
 16. The device of claim1, wherein each photoactive region consists of: a first organic layercomprising a mixture of an organic acceptor material and an organicdonor material; and a second organic layer in direct contact with thefirst organic layer, wherein the second organic layer comprises anunmixed layer of the organic acceptor material of the first organiclayer; wherein the exciton blocking layer is disposed adjacent to and indirect physical contact with the second organic layer.
 17. The device ofclaim 1, wherein each photoactive region further comprises: a firstorganic layer comprising an unmixed layer of the acceptor donormaterial; and a second organic layer in direct contact with the firstorganic layer comprising an unmixed layer of the organic donor material;wherein the exciton blocking layer is disposed adjacent to and in directphysical contact with the first organic layer.
 18. The device of claim1, wherein each photoactive region consists of: a first organic layercomprising a mixture of an organic acceptor material and an organicdonor material, wherein the exciton blocking layer is disposed adjacentto and in direct physical contact with the first organic layer.
 19. Thedevice of claim 1, wherein the device is a photovoltaic device.
 20. Thedevice of claim 1, wherein the device is a photodetector.
 21. The deviceof claim 1, wherein the device further comprises a third photoactiveregion.
 22. A device comprising: a first electrode; a second electrode;a first organic photoactive region disposed between the first electrodeand the second electrode; a second organic photoactive region disposedbetween the first electrode and the second electrode; wherein the deviceincludes a reflective layer, and the first organic photoactive regionand the second organic photoactive region are disposed on the same sideof the reflective layer; wherein λ₁ is the wavelength of one of the topthree absorption peaks of the first photoactive region, and λ₂ is thewavelength of one of the top three absorption peaks of the secondphotoactive region; wherein λ₁ is at least about 10% different from λ₂;wherein the first organic photoactive region is disposed at a positionsuch that there is a maximum in the optical field strength for thewavelength λ₁ within the first organic photoactive region, or within0.05 λ₁/n_(a) of the first organic photoactive region, where n_(a) isthe index of refraction of the material in which the maximum occurs; andwherein the second organic photoactive region is disposed at a positionsuch that there is a maximum in the optical field strength for thewavelength λ₂ within the second organic photoactive region or within0.05 λ₂/n_(b) of the first organic photoactive region, where n_(b) isthe index of refraction of the material in which the maximum occurs.